The recent commercialization of a class of optics known as diffractive optics have enabled some novel ideas to be realized in the area of imaging. Diffractive optics use the wave nature of light to bend and steer it to almost any shape imaginable A simple diffractive lens performs nearly the same function as its refractive counter part (focuses parallel light to a point or forms a real image) but does it in an entirely different way.
A refractive lens bends light in a predictable way as a light ray crosses a surface boundary of differing indices of refraction (a measure of the material's light slowing effect) at an angle to the surface normal. Thus a typical refractive lens has at least one curved surface to direct parallel light rays towards a common focus.
A diffractive lens uses small indentations etched or embossed in the surface of a transparent material to deflect light. This bending is a result of constructively interfering wave fronts (called orders) and is a predictable result of the wave nature of light.
A simple positive diffractive lens has rings of carefully etched grooves of varying width and depth. The grooves tend to get narrower (more dense) towards the edges of the lens because narrower grooves bend light at steeper angles towards the common focus. The lens, however, is more wavelength (color) sensitive than its refractive counterpart. This means light of different colors are focused at different distances from the lens. In normal imaging, this high degree of chromatic aberration is an undesirable effect and is termed "chromatic aberration."
Frequently it is necessary for an optical system to image objects which are located at various distances, at a fixed image plane. To avoid deterioration of the image sharpness as a result of defocusing, an axial adjustment is made to the optical system, which involves moving the entire optical system or any of its optical elements. Various autofocus systems have been developed through the years, which use several defocusing sensing techniques and optical element moving techniques. These are based on at least one moving optical element.
The conventional defocus detection and moving schemes, usually complicate such systems and affects their reliability and cost. The operation of such "autofocus" mechanisms is rime consuming, even with the advent of electronics and of computer processing speed. This can be sometimes detrimental to the operation of the system.
Furthermore, frequently the object surface is not planar or if it is planar, its plane is not parallel to the detector plane. Thus, the autofocus mechanism performance is still limited by the depth of field of the optical system.